Seattle was out of elastic early and I've been making a lot of bias for masks, as I use two yards for each mask.
So you are starting with a square, then cutting that in half, form them to make a pennant or fangs (which ever reference you prefer) and then sew into a tube and cut continuous bias? There are times when you need shorter pieces of bias and yes, just cutting the angles off a half yard of fabric may be the way to go.
Here's a calculator to figure out how big a square you might need:
http://www.quiltersparadiseesc.com/C...Calculator.php
All I know is that one width of fabric (40") is more than I ever need for a queen sized quilt and I cut rather large at that.
So here's the hints that help me -- I just rough cut when I'm making my triangles, typically I lay the fabric out on my ironing board and press the diagonal line.
After I sew that first seam, I use that to true up my rectangle, making sure my first row is full width all the way across. Again, using that straight seam and the markings on my ruler along with the first careful edge, I draw my widths across the fabric.
Only then do I verify the angle on the sides. Using what I have determined to be my straight lines, I position my ruler and trim the sides. If you are ever having problems with excess fabric when you know your rows are correct, the problem is your angle.
I am not afraid to draw on the reverse side of my fabric. Typically I use regular pencil. When I've corrected the edge, I'll go ahead and draw my seam allowance as well, and then I pin the points together.
To help get the offset, there is always one short row near the seam. Trim that down the pencil drawn line about six inches or so, then just line up the rest of the rows. Stick a pin in where your lines/seam line meets and match it up with the other side.
Then it's slowly follow those lines with sharp scissors, making your scissor cuts smooth. You can also carefully rotary cut it, but that is so high stress for me I'd rather use scissors.
With my masks I've been using my leftover quilt bias which is way too big, you really want your raw cuts 1.5", and usually I'm cutting at 3.00". I find I rather like the smooth results I get trimming down my bias. I find it easiest to fold and cut once, rather than trying to trim two long straight sides. So you would start with something wider than 2", fold in half the long way and trim 1" for an unfolded 2".
Any pressing/folding I'm doing I find easier to do in smaller chunks than my big piece of continuous binding. Since I use 36" lengths, I cut the raw binding to that before trying to trim or press or otherwise mess with it.